A heat pump may be reversibly configured to heat or to cool a climate-controlled space. This dual-role capability may allow the heat pump to replace a separate air conditioner/furnace combination. However, because the heat pump uses electricity for both heating and cooling, efficiency (e.g. HSPF) is of utmost importance.
Under some operating conditions, frost may form on heat exchanger (HX) coil used to extract heat from the environment, typically an outdoor coil. Conventional heat pump systems remove the frost using a reverse-cycle defrost, in which the heat pump runs in a cooling mode to defrost outdoor (OD) HX coils with heat transported from indoor (ID) HX coils. The heat produced by the reverse-cycle defrost is lost to the outdoor ambient thus reducing the efficiency of the heat pump. Moreover, supplemental heat consumed to temper indoor air during the defrost adds further to the energy penalty.